1. Field of the Invention
The present invention relates to a method of driving a solid-state imaging device having a CCD register, a solid-state imaging device and a camera having provided with a solid-state imaging device.
2. Description of the Related Art
The number of pixels in a solid-state imaging device remarkably increases in accordance with a progress of recent technologies.
As the number of pixels increases as described above, it is strongly desired to having a function to reduce the number of output data in one frame period according to the need.
As an example of such function, in an electronic still camera, for example, when a user takes a picture, a resolution of a still picture is given a priority so that signals of 500 lines are outputted at a speed of 30 frames/second, for example, from a CCD solid-state imaging device. When a user views an object by an electronic viewfinder, a resolution of a real moving picture is given a priority so that signals of 250 lines are outputted at a speed of 60 frames/second.
However, according to the above-mentioned method, when a user views an object by an electronic viewfinder, signals of remaining 250 lines are useless and discarded.
Therefore, the inventor or the like has previously proposed a solid-state imaging device for obtaining a signal which results from adding signal charges of two pixels distant from each other in the vertical direction in a CCD vertical register (see Japanese laid-open patent application No. 9-55952).
That is, in the above-described solid-state imaging device, as shown in FIG. 1, assuming that Q(k) is a signal charge of kth line, then a vertical register produces an array of signal charges of Q(k)+Q(k+2), Q(k+1)+Q(k+3), Q(k+4)+Q(k+6), Q(k+5)+Q(k+7) . . . on the same time.
Thus, in a CCD solid-state imaging device having a color filter in which two pixels are repeated in the vertical direction, it became possible to add signals of two pixels at the same accumulation timing without discharging signals.
In the above-mentioned solid-state imaging device, it is possible to reduce the number of data in one frame by reducing the number of lines in the vertical direction to ½.
However, in the square lattice pixel, a balance between resolutions in the horizontal and vertical directions is deteriorated.
Also, if the number of data of one frame is further reduced with application of this method, then such balance becomes worse further.
When a CCD solid-state imaging device having 1300000 pixels of 15 frames/second, for example, is operated at 60 frames/second, a resolution in the vertical direction is reduced to ¼.
For this reason, it becomes necessary to reduce the number of data of one frame by reducing the number of data in the horizontal direction to reduce the numbers of data in the horizontal and vertical directions.
Then, there is considered a method of similarly reducing the number of data in the horizontal direction with application of the previously-proposed method of reducing the number of data in the vertical direction.
However, if this vertical direction data reduction method is applied to a reduction of data in the horizontal direction as it is, then as shown by the state in which signal charges are being operated as shown in FIG. 2, blank packets are produced in a packet PH of a horizontal CCD register. Therefore, although the number of data in the horizontal direction may be reduced, the number at which the horizontal CCD register is driven is not changed.
Thus, when the driving frequency of the horizontal CCD register is made constant, regardless of whether or not the reduction of data in the horizontal direction is carried out, one horizontal period becomes the same.
Accordingly, there is not achieved the effect in which the frame frequency is increased by reducing the number of data.
On the other hand, as a method which is used at present to reduce data in the horizontal direction, there are available two methods of (1) method of discharging a part of signal charges in the horizontal direction at a high speed by a horizontal CCD register and (2) method of adding signal charges by a floating diffusion amplifier.
Initially, in the method (1) of discharging a part of signal charges in the horizontal direction by the horizontal CCD register, the horizontal CCD register, for example, is driven, signal charges equivalent to ½ of the number of pixels in the horizontal direction are used as output signals from the CCD register, and remaining ½ signal charges are discharged to a drain of the floating diffusion amplifier unit by driving the horizontal CCD register at a higher frequency.
In this method, since the horizontal CCD register should be driven at a higher frequency, a horizontal CCD register with a high frequency and an excellent transfer efficiency becomes necessary, thereby making a design become more difficult.
Further, since only electric charges equivalent to ½ of the number of pixels are used and remaining ½ electric charges are discharged, it is impossible to reduce the number in which the horizontal CCD register transfers signal charges. This means that the power consumption of the horizontal CCD register becomes twice when a twice frame rate is obtained by discharging ½ electric charges in the horizontal direction, for example.
Also, since ½ signal charges are discharged, an incident light can not be utilized effectively.
Then, because ½ consecutive pixels in the horizontal direction are used, there is then the defect that the imaging range in the horizontal direction is reduced to ½, thereby resulting in a so-called angle of view being reduced to ½.
Also, according to the method (2) in which signal charges are added by the floating diffusion amplifier, by reducing the reset frequency of the floating diffusion amplifier to ½, it is possible for the floating diffusion amplifier unit to obtain an output which results from adding horizontal electric charges of two pixels.
However, since the driving frequency of the horizontal CCD should be increased twice in order to increase the frame rate twice, there is then the defect that the power consumption of the horizontal CCD register becomes twice.
Also, since the driving frequency of the horizontal CCD register and the reset frequency of the floating diffusion amplifier are different from each other, a noise caused by a capacitive coupling tends to be mixed into the solid-state imaging device.
In addition, since the output signal of the pixel signal is separated into a first pixel signal and an added signal of first and second pixels, a sampling possible time due to the output signal being flat is reduced to about ½. Thus, the conventional solid-state imaging device is not suitable as a high-speed solid-state imaging device.